The present invention relates to the switching of optical signals and, more particularly, to an optical switching system that facilitates output power balancing.
In an optical communication network based on Dense Wavelength Division Multiplexing, signals carried on carrier waves of different wavelengths are liable to have different optical powers, for several reasons. One reason is that such a network uses optical amplifiers to maintain signal power. The optical gain of an optical amplifier is not flat, as a function of wavelength. Therefore, even if the incoming multiplexed signals are equal in power, the outgoing multiplexed signals generally are not equal in power. A second reason is that the multiplexed signals typically have different origins, and so have suffered different propagation losses, as a result of having traveled different distances, by the time these signals reach an optical amplifier. If the range of signal powers among the multiplexed signals entering an optical amplifier is too great, the amplifier becomes saturated, resulting in unacceptable data loss.
Two different approaches have been used to solve this problem. The first approach is to flatten the response curve of the system (which is a composite of the response curves of the optical amplifier and of any other wavelength-dependent component, such as filters) by introducing a loss curve that is reciprocal to the response curve. This can be done passively (Y. Li, xe2x80x9cA waveguide EDFA gain equalizer filterxe2x80x9d, Electronics Letters, vol. 31 pp. 2005-2006, 1995) or dynamically (M. C. Parker, xe2x80x9cDynamic holographic spectral equalization for WDMxe2x80x9d, IEEE Photonics Technology Letters, vol. 9 pp. 529-531, 1997; J. E. Ford and J. A. Walker, xe2x80x9cDynamic spectral power equalization using micro-opto mechanicsxe2x80x9d, IEEE Photonics Technology Letters, vol. 10 pp. 1440-1442, 1998). In this approach, the signals remain multiplexed on a common optical waveguide. The second approach demultiplexes the signals to respective channels and attenuates each channel using an optical attenuator.
Optical switches such as 2xc3x972 and 1xc3x972 Mach-Zehnder interferometers can be used as attenuators. FIG. 1 shows a Mach-Zehnder interferometer 10. Interferometer 10 is based on two more-or-less parallel waveguides, an upper waveguide 12 and a lower waveguide 14. Waveguides 12 and 14 are coupled to each other in a first 3 dB directional coupler 16 and in a second 3 dB directional coupler 18. In-between directional couplers 16 and 18, each waveguide 12 and 14 passes through a respective phase shifter 20 and 22. Left end 24 of upper waveguide 12 serves as an input port of interferometer 10. Right end 26 of upper waveguide 12 serves as an output port of interferometer 10. Right end 28 of lower waveguide 14 is an idle port.
The operation of interferometer 10 is as follows. Coherent light entering interferometer 10 at input port 24 is split by directional coupler 16, with half the light continuing rightward in upper waveguide 12 and the other half of the light propagating rightward in lower waveguide 14. Phase shifters 20 and 22 are used to change the relative phases of the light in waveguides 12 and 14. Directional coupler 18 then causes some or all of the light to emerge from interferometer 10 via output port 26 and/or idle port 28, depending on the phase difference, between the light in upper waveguide 12 and the light in lower waveguide 14, that is induced by phase shifters 20 and 22.
FIG. 2 shows the power leaving a specific Mach-Zehnder interferometer 10 via output port 28, relative to the power entering this interferometer 10 via input port 24, in dB, versus the heating power applied to either phase shifter 20 or phase shifter 22. This specific Mach-Zehnder interferometer 10 was fabricated using SiO2 on Si technology, for light of a wavelength of 1.55 microns. Maximum attenuation, of 35 dB, is obtained at point I (approximately 50 mW heating power). Minimum attenuation is obtained at point II (approximately 610 mW heating power). This Mach-Zehnder interferometer 10 therefore is capable of a 35 dB attenuation range. When this Mach-Zehnder interferometer 10 is used as a switch, point I corresponds to the switch being OFF, with almost all power leaving the switch via output port 26, and point II corresponds to the switch being fully ON, with almost all power leaving the switch via output port 28.
The resolution of the attenuation depends on the resolution of the heating power used in phase shifters 20 and 22.
2xc3x972 and 1xc3x972 optical switches also are used as elements in optical switch matrices, such as those taught in PCT application WO 99/60434 and U.S. Pat. No. 6,285,809, for switching optical signals from input waveguides to output waveguides. The present invention is an optical switching system based on an optical switch matrix that combines the switching functionality of optical switches such as Mach-Zehnder interferometer 10 with the attenuation functionality of such optical switches in a single unit.
Therefore, according to the present invention there is provided an optical switching system, for switching optical energy from a plurality of input waveguides to a plurality of output waveguides, including: (a) for each output waveguide: for each input waveguide: at least one respective attenuator for diverting an adjustable portion of the optical energy entering via the each input waveguide to the each output waveguide.
Furthermore, according to the present invention there is provided a method of switching each of a plurality of optical signals, that travel on respective input waveguides, from the respective input waveguide thereof to a desired one of a plurality of output waveguides, including the steps of: (a) providing an optical switch matrix including: for each output waveguide: for each input waveguide: at least one respective attenuator for diverting an adjustable portion of the signal that travels on the each input waveguide to the each output waveguide; (b) selecting the attenuators that divert the optical signals to the desired output waveguides; and (c) adjusting the selected attenuators to balance powers of the optical signals in the output waveguides.
The optical switching system of the present invention is based on an optical switch matrix that includes, for each input waveguide and for each output waveguide, a set of one or more optical switches for diverting an adjustable portion of the optical energy in the input waveguide to the output waveguide. At least one of the optical switches in each set is an attenuator, preferably a Mach-Zehnder attenuator. Preferably, the switches are 2xc3x972 switches. If there are two switches per set, one for input and the other for output, then the input switch has an idle input port and the output switch has an idle output port. The input switch of the last switch set of each input waveguide also has an idle output port, and the output switch of the first switch set of each output waveguide also has an idle input port.
Preferably, the optical switching system of the present invention includes a feedback mechanism for adjusting the attenuators to balance the output powers in the output waveguides. The feedback mechanism includes a power measurement device such as a spectrum analyzer, a set of taps for diverting fixed portions of the optical energy from either the input waveguides or the output waveguides to the spectrum analyzer, and a control unit that receives signals from the spectrum analyzer that indicate the power levels in the tapped waveguides and that adjusts the attenuators on the basis of these signals. Most preferably, each tap includes a directional coupler that is coupled to a respective input or output waveguide.
By xe2x80x9cbalancingxe2x80x9d the output powers in the output waveguides is meant adjusting the output powers in the output waveguides to facilitate the accurate transmission of signals downstream from the optical switching system. Usually, this balancing is done by equalizing the powers in all the output waveguides; but there are circumstances in which the powers are balanced by adjusting them to have mutual ratios not equal to unity. For example, some of the signals may be destined for respective destinations that are farther downstream than other signals. If the powers of all the signals are equalized, then, because signal attenuation varies in the same sense as distance traveled, the signals with distant destinations arrive at their destinations with lower powers than the signals with nearby destinations. In that case, it often is desirable to adjust the powers of the signals with distant destinations to higher levels than the powers of the signals with nearby destinations, so that all the signals arrive at their respective destinations with equal powers.